I watched John Carter straight after reading A Princess of Mars, which is always a bad idea. Don't ever do that. It's why I didn't like the film at all, they mixed up events in the book (for time, I suppose) and nothing was even remotely similar to my imaginary version (except maybe the sky).
It's a shame I can't judge it without thinking of the book, because there were enough changes to pass off as only being inspired by the book(s). Lol.
Visually, it was very well done.